I can't locate any caps that aren't plastic. The other possibility would be the rubber EZ Lube plugs if the dimension of the hole in your cap is similar.
Lost one of mine, took the other one off. Having them off I can feel the hub when I stop for gas or a bathroom break. Don't need an overheated axle bearing.
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Steve, Michele and Ginger
2012 Tacoma crew Cab Pre-Runner
Tow package, Scanguage, Timbrens
2016 Feather 7 18RBM
If this one works for you, it is metal...darn things are so expensive!
I found this one that is relatively inexpensive. I got it two months ago.
The plastic ones don't last very long.